This invention relates to exercise and rehabilitation devices and, more particularly, to weight training machines.
A typical weight machine comprises a stack of weight plates which travel along guide bars and which are coupled to a lifting mechanism which translates a user's exercise movements to raising the stack from a resting, unloaded position and then lowering the stack back down to the unloaded position. The user typically moves a selected plurality of the weight plates over a fixed distance, and the resistance to the user's exercise movements created by the weight plates is used to strengthen specific muscle groups. The user must change the number or size of weight plates coupled to the lifting mechanism in order to vary the amount of resistance to a given movement by the user. That is, the user must increase the number or size of weight plates to increase resistance to a given movement, and vice versa. The range of travel for a given amount of weight does not vary with the amount of weight selected.
Known weight training machines have many disadvantages. First, high speed training is hampered by inertial "fly away" effects of the weights, especially at low weight selections. The fly away effects of the weights create irritating noise and nonuniform resistance. The nonuniform resistance results from the fact that the effective resistance that the weights exert against the user's movement varies as the velocity of the weights varies. Many exercises are most efficient to develop particular types of strength if the user moves quickly from his or her initial position to the fully flexed or extended position. In such exercises the velocity of the user's movement and, therefore, the velocity of the weights may change significantly during a single movement. Because the kinetic energy of the weights is proportional to the square of their velocity, the resistance of the weight to the user's movement may vary over a wide range. Indeed, resistance may be zero if the weights become airborne during the movement. Second, the available weight increments are determined by the size of the individual weight plates which make up the weight stack. If small weight increments are desired, then numerous small weight plates must be used to accommodate stronger users, and this results in a very large weight stack. On the other hand, if large weight increments are desired, then large weight plates must be used, and all users must cope with large increment values. Third, a substantial amount of friction results when the weight plates travel along and rub against the guide bars, and the friction interferes with mid-motion stops and starts. Fourth, known weight machines cannot by their nature accommodate zero-resistance or very low load exercises since they do not provide for counterbalancing of the lifting mechanism.